Many orthopaedic procedures involve the implantation of prosthetic devices to replace badly damaged or diseased bone tissue. Common orthopaedic procedures that involve prosthetic devices include total or partial hip, knee and shoulder replacement. For example, a hip replacement often involves a prosthetic femoral implant. The femoral implant usually includes a rigid stem that is secured within the natural femur bone tissue. The femoral implant further includes a rounded head that is received by, and may pivot within, a natural or artificial hip socket. Knee replacement is somewhat similar, and typically includes one or more implants that have both bearing surfaces and stems.
Total hip replacement procedures typically involve the implantation of two main component systems: the femoral component (as discussed above) and an acetabular component. The femoral component is anchored within the existing femur and includes a head that replaces the natural hip joint femoral head. The acetabular component is secured within the acetabulum of the patient and serves as a bearing surface for the femoral component.
Many acetabular cups include an outer shell component and an inner liner. The outer shell component has an outer dimension configured to fit within the acetabulum of the patient. The outer shell is typically formed from a high strength alloy, such as a titanium alloy, in order to withstand the pressures exerted on the hip joint during normal activities. The inner liner is configured to tightly fit within the acetabular outer shell component. The inner liner serves as the bearing surface for the femoral head. Accordingly, the inner liner is typically constructed of a polymeric material, such as for example, polyethylene. Inner liners may also be constructed of cobalt chrome or ceramic material.
The acetabular component of hip replacement includes a number of sizing and shape considerations. In particular, the outer diameter of the outer shell is configured to be received by the patient's acetabulum. While the acetabulum may be reamed and otherwise prepared to receive the outer shell, it is still necessary to provide multiple sizes of outer shells to accommodate the varied anatomies of different patients. In addition to the outer diameter of the outer shell, the inner diameter/geometry of the inner liner must be configured to receive the femoral head (prosthetic or otherwise) and allow a suitable range of motion. The inner diameter and geometry of the inner liner can often define 10-15 different styles.
Typically, the ultimate determination of which outer shells size and which inner liner style to use occurs during surgery. In particular, the surgeon usually first performs a trialing procedure in which one or more prosthetic devices are temporarily implanted. The trial devices are evaluated and then the final prosthetic device(s) is selected based on the evaluation of the trial devices.
During the trialing process, the surgeon assesses the acetabulum and the femoral head and attempts to select the correct combination of outer shell size and inner liner style. The outer shell is typically selected based on the geometry of the acetabulum identified by the surgeon. An inner liner must thereafter be selected. The inner liners are available in different sizes and styles. As discussed above, the size of the inner liner ultimately depends on the size of the femoral head. Different liner styles depend on patient geometry and can affect the range of motion. Examples of known liner styles include neutral, 10°, lateralized, and lipped. Each is appropriate for a particular situation.
Accordingly, in order to select the appropriate components for the acetabular implant, the surgeon implants trial components on a trial and error basis until a suitable combination of outer shell size and inner liner style provides acceptable results. To this end, surgeons must have available to them outer shells of various sizes and corresponding trial inner components of different styles and sizes. Moreover, in order to provide maximum flexibility, all possible styles and sizes of inner liners should be available for every possible size of outer shell component.
It can be readily be appreciated that providing inner liners having all of the possible configurations for each of the different size outer shell components can require a large number of trial components. For example, if there are six outer shell sizes and thirteen inner liner configurations, then up to seventy-eight inner liner trial components must be provided to the surgeon, thirteen styles for each of six outer shell sizes. Providing such a quantity of inner liner trial components in addition to six outer shell sizes is both costly and inconvenient to manipulate in the surgical environment.
One prior art patent, U.S. Pat. No. 5,879,401 to Besemer et al., which is incorporated herein by reference, teaches an acetabular trial system that in theory can reduce the number of trial liner components that are necessary to cover various outer shell sizes and inner liner styles. To this end, U.S. Pat. No. 5,879,401 teaches the use of outer shell components that have uniform inner diameters. Because the inner diameter of the outer shell is uniform regardless of its outer diameter, only one set of inner liners is necessary.
One drawback of such a design is that it requires the outer shells to have widely varying thicknesses. In particular, because the inner diameter of the outer shell remains the same while the outer diameter varies, the outer shell thickness must vary accordingly. In the case of the largest diameter outer shell, the thickness of the outer shell could well approach 16-20 mm in thickness. While such a design is possible, it has a number of drawbacks. One drawback relates to the use of final outer shell components in the trial reductions of the joint.
More specifically, because the outer shell may typically be selected prior to the trial reduction of the joint, surgeons often elect to implant the final outer shell, and not the trial outer shell, prior to performing the trial reduction using the trial inner liner. To permit the flexibility of using either final outer shell or the trial outer shell during trial reduction, the trial outer shell and the final outer shell must be substantially identical in dimensions. Moreover, if this flexibility is to be provided using the method of U.S. Pat. No. 5,879,401, then the final outer shell must, like the trial outer shell, be available in varying thicknesses in order to maintain the constant inner diameter. Thus, in the case of the largest outer shell sizes, the final outer shell can also require a thickness approaching 20 mm. However, normal outer shells have a thickness of on the order of 5 mm-8 mm.
It is undesirable to have such thick outer shells because thick outer shells require the use of correspondingly thinner inner bearing or liner devices. Thinner bearings or liners are undesirable because joint longevity increases as a function of liner thickness. In particular, because the liner serves as the bearing surface for the humeral head, a thicker liner will provide a bearing surface that can withstand greater wear. As a consequence, it is desirable to provide the final liner with larger thickness. Thus, if excess thickness is used for the outer shell, that excess thickness represents thickness that could have been used to increase the liner thickness and thus the longevity of the joint.
Thus, there are a number of drawbacks to varying the thickness of the outer shell component to accommodate various inner liner trials.
Accordingly, there is a need for a modular acetabular trial system and method that provides flexibility of outer shell sizes and inner liner styles with a reduced number of components, and which optionally allow the surgeon to employ final outer shell components during the trial reduction.